This invention relates to a method and apparatus for supplying gas, preferably under pressure, to a vacuum drum device for maintaining the passages thereof clear from blockages, and more particularly to a method and apparatus for supplying gas to the vacuum ports of a rotating vacuum drum in an apparatus for the manufacture of plastic bags or containers.
In the production of individual flexible web products such as plastic containers and bags, the bag stock is typically supplied in the form of a continuous web of thermoplastic material which has been folded upon itself to form two plies. In forming individual bags, portions of the thermoplastic material, such as polyethylene, are severed from the web. These severed areas become side seams for the bags and are typically sealed at the same time as they are severed by the use of a heated wire element. The bags are then stacked, counted, and packaged by packing equipment.
The severing and sealing operation typically takes place on a relatively large diameter rotating product drum which may contain multiple heated wire severing and sealing elements positioned in grooves located within the outer periphery of the drum. As the drum rotates, different severing and sealing elements are actuated to raise them up to the drum surface to sever and seal a respective portion of the bag stock web which is secured to the drum surface by seal bar assemblies. The individual bags are retained on the product drum by a vacuum arrangement as the drum rotates.
Typically, the vacuum arrangement includes a number of surface vacuum ports which communicate with a source of vacuum. As the individual bags are formed from the continuous web, a spacing between successive bags is created resulting from the melt back of the thermoplastic web material as the side seams are severed and sealed by the heated wire element. During the severing and sealing operation, some smoke and particles are formed from the melted plastic and from the degradation products of the melted plastic. Additionally, long, thin filaments (angel hair) of plastic may be formed as the seal bar assemblies are pulled away from the drum surface. The still soft plastic bead formed by the severing and sealing operation may tend to stick to the seal bar assemblies and be drawn to form the plastic filaments.
Individual bags are then taken from the drum, stacked, and packaged. Presently, individual bags are taken from the drum by a smaller transfer drum, also suitably equipped with vacuum capabilities, including surface vacuum ports. The vacuum on the bags on the large drum is relieved at an appropriate point, and the bags fall onto the smaller drum where they are held in position by vacuum. At an appropriate point, the vacuum is released and the individual bags are pulled off the smaller drum by an orbital packer or similar device.
Because of the vacuum being pulled at ports located on the surfaces of the product and transfer drums, smoke, particles, filaments, and other contaminants from the severing and sealing of the individual bags are drawn into the ports where the particles become lodged on the surfaces of the vacuum ports and vacuum passages to form a waxy solid. Over time, the waxy buildup constricts the vacuum ports and passages and leads to misalignment problems with the bags on the drums as insufficient vacuums are applied to the bags. The misalignment of the bags in turn may lead to stacking and packaging problems and equipment jams. Additionally, some of the contaminants may be drawn through the vacuum passages and into the vacuum pump equipment causing maintenance problems there.
Periodically, the equipment must be shut down entirely, and a time-consuming and laborious cleaning and maintenance of the drum vacuum ports and passages must be carried out. The drums must be taken apart and the passages cleaned using scraping devices and solvents. This is expensive not only from the standpoint of labor, but also because of lost production due to the down time of the equipment.
In the past, changes to the construction of the bag making equipment have been made to improve access to the vacuum passages and ports in the drums. While making disassembly and cleanup of the drums easier, these changes did not address the problem of wax buildup. Additionally, manual intermittent air blasts have been used after shutting down the equipment in an attempt to blow accumulated particles from the vacuum passages of the transfer drum. This was not found to be very effective in dislodging the waxy particles which adhere to the vacuum passage walls. Again, the problem of the building up of accumulated particles and debris was not addressed.
Accordingly, the need still exists in this art for a method and apparatus which substantially reduces or eliminates the problems of waxy solid particle buildup in the vacuum passages of plastic bag making equipment and maintains those passages clear from blockages.